If a person intends to do something, and an obstacle prevents the completion of that action, is its incompletion considered to be by their will or not?

The belief of the Ahl al-Sunnah wal-Jama'ah is that human beings possess will and ability, which are subservient to the will and power of Allah. If someone resolves to commit a sin and takes steps towards it, but is then prevented from committing it by an impediment, they are held accountable for their resolve. In contrast, if someone abstains from sin by their own will out of fear of Allah, they are rewarded. As for one who contemplates sin but does not commit it, scholars differ on accountability. Some hold them accountable if there was determination and resolve, while others believe accountability applies to a fleeting thought that does not settle. The accountability for resolve is evidenced by the hadith: "If two Muslims confront each other with their swords, both the killer and the killed are in the Hellfire," because the one who was killed was eager to kill his companion.